For the measuring of the mass flow of a flowing medium there are known instruments which determine a Coriolis force acting upon the flowing medium. This force is an inertial force which acts in addition to the centrifugal force upon a mass point which moves in a reference system that is moved as such on a circular path. A measuring instrument suitable for flowing media can consists, for example, of an oscillatory U-shape tube loop whose in- and out-flow ends are clamped in a massive pillow block. When this U-shaped tube loop is vibrated at its arc end in a direction vertical to the plane of the tube loop, the Coriolis forces of the through-flowing medium acting on the two legs of the U-shaped tube loop cause to-and-fro distortions of the U-shaped tube loop around an axis which lies centrally between the two legs and parallel to them. The extent of these distortions is determined by means of suitable sensors which are arranged in the region of the two legs and from whose measuring signals the flow amount is calculated. Such a device is known, for example, from U.S. Pat. No. 4,187,721 and exhibits a U-shaped tube and a T-shaped leaf spring which forms the arms of a tuning fork. The T-shaped leaf spring carries at its free end a vibration generator which acts upon the freely oscillating end of the U-shaped tube so that the U-shaped tube and the leaf spring oscillate in opposite directions with a characteristic frequency like a tuning fork. From DE-Patent 2 822 087 it is known to provide two U-shaped tube loops through which the medium flows consecutively in the same direction in order to augment the sensitivity of the gauge. The vibration generator is mounted at the free end of the U-shaped tube loops and the sensors measuring the distortions are affixed to the two U-shaped tubes. Of a similar design are the constructions described in DE-A 2 938 498, DE-A 3 007 361 and DE-A 3 046 793.
On the aforementioned constructions are employed sensors such as light switches or the like, which trigger switching signals when the mechanically sensor parts are passing the zero position. In an electric circuit the time difference between the zero crossings of the lateral sensors is determined. This time difference is directly proportional to the mass flow of the medium flowing through the U-shaped tube loop. EP-A 0 083 144 shows the use of sensors which transform the mechanical vibrations into analogous sinusoidal measuring signals from which is likewise determined, by means of an electronic switch, a time difference proportional to the mass flow.
These prior constructions are not frequency-selective. Superposed interfering signals from turbulences in the fluid flow or movement of the tube line can strongly disturb the sinusoidal course and thereby the measuring of the time displacement between the zero crossings. The employment of electronic filters in the two signal channels is generally not a solution to this problem because the time or phase displacements are very small (of the order of a few microseconds) and can be strongly falsified, e.g. by varying thermal drifts of the two filters. Finally, because of the relatively low oscillation-frequencies employed (i.e. between 60 and 100 Hz) and only two measurements that may be taken per period, this result is a maximum of about 200 measurements per second and therefore, the information content of the measuring signals not fully utilized.